Gas-or vapor-filled electric discharge lamps



July 12, 1960 w. MEYER 2,945,146

GAS- OR VAPOR-FILLED ELECTRIC DISCHARGE LAMPS Filed Feb. 10, 1959 I'Jlllllllllllllllb Unite GAS- OR VAPOR-FILLED ELECTRIC DISCHARGE LAMPS Wilfried Meyer, Berlin-Charlottenburg, Germany, as-

signor to Patcut-'Ireuhand-Gesellschaft fiir elektrische Gluhlampen m.b.H., Munich, Germany This invention relates to gasor vapor-filled electric discharge lamps and more particularly to high and super pressure lamps of this type which are characterized, among other features, by the production of a widely spread spectrum extending from ultravioletto infrared, which provide extreme brightness, and which have small dimensions.

Because of these properties a highand super pressure discharge lamp is suitable for many purposes. Often the envelope of'this discharge lamp is provided with a reflecting coating which reflects any radiation (which is not emitted to the objects to be illuminated but to the opposite half-area of the lamp) to the objects themselves to be illuminated or, into the discharge path.

In a high and superpressure lamp having spaced electrodes with, a short discharge are therebetween and an envelope shape in which the reflecting coating throws back into the discharge path any radiation falling on such coating, it has been found that the electrodes are subjected to the reflected radiation. This electrode irradiation is, caused first because the discharge are partly encloses the electrodes and, secondly because an envelope serving as a reflector does not provide an exact optical shape. The electrodes consisting of a refractory metal, such as. tungsten in the compact state, are undesirably heated by means of IR-radiation (infra-red radiation) whichemanates from the discharge and the additional IR-radiation which is reflected by reflecting coating, so that sputtering of the electrodes is increased and lamp life is decreased by enhanced bulb blackening.

A further disadvantage in this type of highand superpressure lamp having a reflecting coating is that the States Patent filling gas absorbs a considerable portion of the reflected IR-radiation, thereby adding heat to the filling gas. As a result of this additional heating of the filling gas, the operating temperature of the lamp wall may be increased by about 12%. Owing to the fact that normal wall temperatures in highand superpressure discharge lamps are already very high, the additional heating of the filling gas may exceed the safe maximum limit of wall temperature and may lead to a softening of the envelope wall.

In order to prevent any chemical reaction between the reflecting coating and the filling gas, the reflecting coating is suitably applied to the outer side of the bulb body. In the case of a conventional metallic reflecting coating, IR-radiation would, in such an arrangement, penetrate the wall of the bulb body twice, thereby further heating the wall to an undesirable high temperature.

It is an object. of the present invention to avoid and overcome the foregoing and other difliculties of and objections to the prior art highand superpressure discharge lamp having a reflecting coating by providing the discharge lamp with a spherical bulb body and a reflecting coating adapted to reflect the visible radiation falling upon the reflecting coating into the discharge path, the reflecting coating being made permeable to the IR-radiation emitted by the discharge, thereby preventing the overheating of the electrodes and filling gas.

means Patented July 12, 1960 Another object of the present invention is to provide a highand superpressure discharge lamp having a reflecting coating on the outside of the bulb body, which reflecting coating is permeable to IR-radiations so that such undesirable IR-radiations only pass through the wall of the bulb body once.

The aforesaid objects of the present invention, and other objects of the present invention which will become apparent as the description proceeds are achieved by applying the reflecting coating to a hemispherical half portion of a nearly spherical bulb body in which the center of such hemispherical portion lies nearly in the center of the discharge and the circumscribing plane of the great circle of the hemispherical portion passes through the discharge axis.

For a better understanding of the present invention reference should be had to the accompanying drawing wherein like numerals of reference indicate similar parts and wherein the sole figure is a side-elevational view of a superpressure mercury discharge lamp, partly in vertical section.

With specifiic reference to the sole figure of the drawing cylindrical tapered electrodes consisting of thoriated tungsten are indicated by the reference numeral 1. A quartz spherical bulb body 4 is provided with tubular extensions 5 for electrode inleads 3. The center of the spherical bulb body 4 coincides with the center of the discharge between the electrodes 1. Half of the spherical bulb body 4 is provided with an outer reflecting coating 6 which reflects the visible radiation from the discharge but which is, however, permeable to IR-radiation, A

circumscribing end line 7 or great circle of the hennspherical reflecting coating 6 (a short length of which is visible on the back of the bulb body 4 between the electrodes 1) lies in a plane passing through the discharge axis so that all ofthe visible radiation of the lamp is emitted or reflected back into the uncoated hemispherical left-hand portion of the bulb body 4. The IR-radiations falling on the coated right-hand hemispherical portion penetrate, however, through the reflecting coating 6 so that such IR-radiations are not thrown back through the wall of the bulb body 4 into the discharge path, into the mercury yapour and upon the electrodes 1, thereby eliminating the heating oftthe wall, the vapor filling and the electrodes in an undesired manner by the IR-radiatlons.

The mercury-vapor pressure in this lamp may amount in operation to 35 to atm. xenon or any other heavy rare gas maybe admixed to the mercury filling. The invention may also be used in a lamp filled only with rare gas (i.e. a xenon high-pressure discharge lamp).

Germanium or silicon, as well as sulphide of antimony or selenium, are quite suitable as the material for the main layer of the reflecting coating 6. These materials have a high-reflecting power for visible radiation and a great permeability to IR-radiation. Very good results have been obtained with a germanium main layer which was deposited by thermal evaporation and has a thickness such as some tenth millimeters so that it will transmit about 1%-30% of the IR light rays of about 435 millimicrons which are incident upon it. The reflecting power of such main layer for visible radiation may be increased by means of interference filter films.

In order to provide such an interference filter, a film of sulphide of zinc, aluminum, titanium or tin oxide is first applied to the outer side of the right-hand hemiindex of refraction than that of the first interference filter film. Magnesium fluoride or any other suitable metallic fluoride may be used for the second film. The main layer of germanium or silicon may then be applied directly to this pair of interference filter films. In order to further improve the reflecting power of thernain layer for visible rays this pair of first and second films may first be covered with a similar pair or with a double pair of interference filter films of difierent indexes of refiractions and composed of the aforesaid materials, whereupon such additional pairs of films may be covered with the thicker main layer of germanium or silicon. Each of the pairs of films are formed of different material having respectively diiferent refractive indexes, the lower index film of each pair having a refractive index lower than that of all films adjacent to it. The film in contact with the main layer has a higher index than its adjacent film.

The optimum optical thickness of the main layer may be ascertained from the radiation spectrum of the discharge lamp in question to provide high reflection for visible radiation, and high permeability of IR-radiation. This main layer may be provided also with a protective layer such as vaporized silica which protective layer, of course, must also be permeable to IR-radiation.

While in accordance With the patent statutes one bestknown embodiment has been illustrated and described in detail, it is to be particularly understood that the invention is not limited thereto or thereby.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electric discharge lamp comprising a sealed transparent bulb, an ionizable atmosphere having a high pressure and superpressure during the operation of the lamp, spaced electrodes with a short discharge are therebetween, a reflecting coating on the outside of a hemispherical bulb portion, the center of such hemispherical portion being in the center of said discharge arc, the circumscribing plane of the great circle of said hemispherical portion passing through the axis of said discharge arc, said coating being permeable to IR-radiation emitted by said discharge are in order .to prevent any overheating of said electrodes and of said ionizable atmosphere, said coating consisting of a semi-transparent reflecting main layer of germanium and means for .increasing its reflectivity for visible light rays, said germanium layer "having a thickness such that it will transmit about 1% to 30% of the light rays of about 435 millimicrons being incident upon it, said means for increasing its reflectivity comprising at least one pair of superposed films of material deposited on the outside of said bulb portion, said superposed films having alternately higher and lower refractive indices than their adjacent films respectively, the film in contact with said bulb being formed of zinc sulphide and having a higher refractive index than its adjacent film, said adjacenl'film being formed of a metallic fluoride, said two films representing one pair of said superposed films, further pairs of said superposed films being formed in the same mannet and of the same materials, each of said films having an optical thickness of the order of one fourth of the wave length of light rays in the spectral region of about 500 to 600 millimicrons.

2. An electric discharge lamp comprising a sealed transparent bulb, an ionizable atmosphere having a high pressure and superpressure during the operation of the lamp, spaced electrodes with a short discharge are therebetween, a reflecting coating on the outside of a hemispherical bulb portion, the center of such hemispherical portion being in the center of said discharge are, the circumscribing plane of the great circle of said hemispherical portion passing throngh the axis of said 'discharge are, said coating being permeable to IR-radiation emitted by said discharge are in order to prevent any overheating of said electrodes and of said ionizable atmosphere, said coating consisting of a semi-transparent reflecting main layer of germanium and means for increasing its reflectivity for visible light rays, said germanium layer having a thickness such that it will transmit about 1% to 30% of the light rays of about 435 millimicrons being incident upon it, said means for increasing its reflectivity comprising at least one pair of superposed films of material deposited on the outside of said bulb portion, said superposed films having alter nately higher and lower refractive indices than their adjacent films respectively, the film in contact with said bulb being formed of zinc sulphide and having a higher refractive index than its adjacent film, said adjacent film. being formed of a magnesium fluoride, said two films representing one pair of said superposed films, further pairs of said superposed films being formed in the same manner and of the same materials, each of said films having an optical thickness of the order of one-fourth of the wave length of light rays in the spectral region of about 500 to 660 millimicrons.

Friederich Oct. 31, 1939 Biggs Nov. 28, 1939 a. Li 

